Method of well logging



United States Patent Office 3,037,115 Patented May 29, 1962 of Delaware Filed Oct.14, 1954, Ser. No. 462,370

No Drawing.

4 Claims. (Cl. 250-435) This invention relates to a method of neutron logging,

of wells to deter-rninetherein point-s ofentry of different fluids.

When different fluids enter the producing stream of a well and the presence of one of said fluids in the final output of the well is undesirable, it is possible to elimimate the entry of the undesired fluid if the depths at which it enters the well bore can be accurately determined. Quite often oil and brine are'both present in the producing stream of an oil well. It is desirable that the output of the well be as rich in oil as possible so that the well pumping operation will be eflicient from the standpoint of producing oil, and so that the need for separating operations will be reduced. If the depths at which brine enters the stream of the well can be accurately determined; then by well known methods the walls of the well can be cemented over at these depths thus reducing the amount of water contaminating the well stream.

The determination of points of entry of different fluids into a well presents several distinct problems.

Soon after the different fluids enter the well bore they begin to mix. Therefore, the samples of the admixture in the well stream must be large enough so that a fair representation of the relative quantities of the different fluids entering at that level is obtained. However, the samples analyzed must not be too large because the purpose of analyzing samples is to find distinct depth ranges wherein each fluid enters the bore. If the samples are too large, i.e., are taken along too great a length of the well bore, accurate location of the interfaces between said ranges is impossible.

It is the principal object of this invention to provide a method of logging a well to determine therein points of entry of different fluids.

It is a more specific object of this invention to provide a method of logging oil wells to determine points of entry therein of brine and oil.

'Further objects and advantages of my invention will appear more fully throughout the following specification and claims wherein the specific application of my invention to oil wells is described.

Broadly stated, my invention comprises:

The method of determining the respective points of entry into a well of different fluids which significantly differently affect a neutron stream which comprises progressively establishing in local areas in the well a neutron stream and making a log indicative of variations in a parameter of the aflected neutron stream at a fixed distance from its source.

When a mixture of certain fluids is bombarded by neutrons, and when the neutron stream has the proper energy level, the different components of the mixture may affect the neutrons in various ways. Some substances will scatter neutrons. Some substances will retard the neutrons velocity. Some substances will absorb neutrons, and some substances will exert combinations of these effects on the neutrons. When a mixture of several substances is bombarded by a stream of neutrons it is possible to readily determine the relative amounts of the various substances present in the mixture if the effects they produce on a neutron stream are significantly different. Thus, when a neutron stream of a given energy level bombards a mixture of brine and crude oil and the neutrons reflected by or transmitted throughthe mixture are then detected, the intensity of the neutron stream detected willvary as the relative amounts of brine and crude oil in the mixture vary.

When a method is used which utilizes'a neutron stream, care must be exercised as to the energy level of the neutron stream used, because, as is well known in the art, the' effects produced on neutrons by, different substances vary. Thus neutrons of.

greatly with the energy of the neutrons. a very high energy level may pass unaffected through a certain mixture whereas neutrons of. a slightly lower energy level are affected selectively by the different components of such mixture- The determination of the levels wherein oil and brine enter the producing stream of an oil well is accomplished by the present invention by detecting the above mentioned differences in the eflect by oil and brine inscattering, retarding, and absorbing a neutron stream. These differences are most pronounced when theenergy level of the neutron stream at the point where it starts traversing the sample is approximately 10,000 electron volts.

The apparatus which may be used in carrying out the method of this invention may consist of a wellknown form of tubular well surveying tool, commonly referred to as a sonde, but 'which contains in fixed spaced relation a source of neutrons enclosed in a suitable shield and a detector of slow neutrons. The detector is connected to a suitable devicewhich records the detected signalsrelative to the depths at which they originated. The neutron source and shield are mounted in the bottom of the sonde. Just above the shield, means are provided for the passage of well fluids into the sonde. The slow neutron detector should be mounted in the sonde from 6 to 12 inches above the shield so that it detects the neutron stream that has been transmitted for that distance through the fluids between the shield and the detector.

The particular detector used is not critical. Its function is to detect only slow neutrons. Examples of suitable detectors are a boron coated proportional counter and a 13-10 enriched boron-trifluoride counter.

The particular neutron source used is not critical. Examples of suitable sources are the products resulting from the action of a radioactive element on one of the following: aluminum, beryllium, boron, lithium, magnesium, phosphorus, and sodium. The most likely used sources will be mixtures of radium-beryllium or poloniumberyllium.

The shield should be of such thickness and composition that the neutron stream entering the fluid mixture has an energy level of about 10,000 electron volts. It may be made of lead or a suitable plastic.

In operation, the sonde is lowered into the well so that the fluids produced by the well, substantially at the levels where they enter the well bore, may flow into the sonde just above the shield. The fluids then flow through the sonde upwards around the detector and out of the top of the sonde. As the fluids produced by the well pass through the neutron stream emanating from the shield, they scatter, absorb and retard the neutrons of said stream. The degree to which the brine produced by the well affects the neutron stream is sufficiently distinct from the degree to which the oil afiects the neutron stream that when an unusual amount of oil or water is present at any point which the sonde passes, the recording device connected to the detector of slow neutrons will indicate a rapid change in one direction or the other. The sonde is lowered throughout the well while the recording device makes a trace of the detector signals on a depth log.

As a separate operation the sonde should be placed in a flask of brine which has been separated from the well output, and the effect of the pure brine on the neutron stream recorded. Similarly, the sonde should be placed in a flask of substantially pure oil separated from the well output, and the eflect of the pure oil on the neutron stream recorded. These two records can then be used to calibrate the depth log, to directly indicate quantitatively the relative amounts of brine and oil present in each local area throughout the well.

It is not essential that the apparatus of this invention be constructed specifically as described above. Rather than measuring the intensity of the neutron stream transmitted through the well fluids, similar accurate information can be obtained by measuring the intensity of the neutron stream reflected by the well fluids.

This method of locating points of water entry into wells producing oil and water has several pronounced advantages.

The signal detected by the detector of slow neutrons is independent of the way in which the oil and water are mixed.

Furthermore, this method indicates the points of entry of the water and the points of entry of the oil. Because the points of fluid entry are so indicated, that is, because the points of oil entry are also known, the chance that the oil output of the well would be reduced by subsequent cementing operation, is substantially eliminated.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I, therefore, particularly point out and distinctly claim as my invention:

1. A method for determining the location of respective areas of entry of different fluids into a well bore, each of said fluids having a significantly different effect upon a stream of slow neutrons, which comprises emitting a Stream of slow neutrons at different locations along the Well bore, passing the fluids produced by the well through the neutron stream, and logging, at a fixed distance from the source of said slow neutrons, the variations at each location in that parameter of said slow neutron stream which has passed directly through and been affected by the well fluids at said location.

2. The method of claim 1 characterized further in that the energy level of said slow neutron stream is approximately 10,000 electron volts.

3. The method of claim 1 in which the slow neutron stream is transmitted through the fluids for a distance of approximately 6 to 12 inches.

4. The method of claim 3 characterized further in that the energy level of said neutron stream is approximately 10,000 electron volts.

References Cited in the file of this patent UNITED STATES PATENTS 2,335,409 Hare Nov. 30, 1943 2,463,733 Albaugh Mar. 8, 1949 2,544,412 Bird Mar. 6, 1951 2,726,338 Goodman Dec. 6, 1955 2,735,944 Greer Feb. 21, 1956 2,738,426 Hurst Mar. 13, 1956 2,744,199 Juterbock et a1. May 1, 1956 2,813,980 De Witte Nov. 19, 1957 2,926,259 Dewan Feb. 23, 1960 

